1. Field of the Invention
This invention relates to an interface circuit for inputting a digital signal to a bipolar integrated circuit, and more particularly to an interface circuit used for TV video reception processing.
2. Description of the related art
A video reception circuit of Improved Definition TV (IDTV) is constructed as shown in FIG. 1, for example. In the video reception circuit, tuner 11 detects a modulated video signal from a reception signal supplied via antenna 10, A/D converter 12 converts the modulated video signal into a corresponding digital signal, and video processor 13 demodulates the modulated video signal derived from A/D converter 12. The demodulation process includes a process for separating the digital video signal into original color signals of red, green and blue and a process for storing the digital video signal in a memory for each frame unit, for example. D/A converter 14 converts an output signal of video processor 13 into a corresponding analog signal which can be displayed on a display unit. Each function block is formed in the IC configuration and is connected to a corresponding function block.
In a case where video processor 13 and D/A converter 14 are respectively formed as MOS IC and bipolar IC, an interface circuit shown in FIG. 2 is formed in the chip of D/A converter 14, for example. The interface circuit includes bipolar transistors Q1 to Q7, resistors R1 to R5 and constant current source I1, and is connected to video processor 13 via line N1. CMOS circuit 21 constitutes the output section of video processor 13, and sets the potential of line N1 to ground potential level (=0 V) or VDD level (=5 V) according to digital signal DA1. The interface circuit detects the potential of line N1 and supplies output voltage OUT corresponding to the detected potential to D/A converter 14.
When the digital signal is supplied via line N1 which lies outside the MOS IC and bipolar IC chips, harmonics are generated from line N1. When the amplitude of the digital signal is as large as 5 V, the operation of tuner 11 is disturbed by the harmonics generated from line N1, significantly degrading the picture quality.